Greater than 80 percent of all U.S. electric power is currently generated from steam turbine cycles. Power cycle steam turbines process heat generated from combustion of hydrocarbon fuels (e.g., coal, natural gas) in a burner through a thermodynamic power cycle and convert the heat into useful work. The combustion of hydrocarbon fuels occurs at high temperatures, which may exceed 1900° C. However, steam turbine blades are exposed to high centripetal accelerations and may begin to creep and/or mechanically deteriorate when coupled with temperatures even significantly lower than the turbine blade melting point.
To protect the heat-sensitive turbine blades, higher mass flow rates of water are typically utilized in a burner heat exchanger to convert heat from the burner into a lower temperature (<650° C.) steam as compared to the combustion gas temperatures. As a result, steam engines often have lower theoretical Carnot cycle thermodynamic efficiencies as well as real thermodynamic efficiencies than could be achieved if the highest temperature of the cycle was closer to the temperatures of the combustion gases in the burner. Typically, thermodynamic efficiencies for converting heat into useful work (e.g., electricity) in these conventional steam turbine cycles is less than 50 percent and may be more commonly 35-42 percent.
Some existing steam applications combine two or more thermodynamic cycles to increase thermodynamic efficiencies. For example, Combined Cycle Gas Turbine (CCGT) systems have achieved cycle efficiencies of ˜60% utilizing a gas turbine Brayton thermodynamic cycle for the “topping” cycle (i.e., the first highest temperature cycle in the overall thermodynamic power conversion system). However, these gas turbine “topping” cycles operate from combustion of a gas, and thus require either a gas fuel source or additional steps for producing gas fuels from solid hydrocarbon fuels.
It is theoretically possible to enhance the overall thermodynamic power cycle efficiency using a “bottoming” cycle (i.e., the lowest temperature cycle in the overall thermodynamic power conversion system) that operates from waste heat of turbines. However, such cycles are typically uneconomical because of very large, expensive equipment needed to extract energy from the small temperatures differences between condensing steam and outside air and/or water. This infrastructure and operating cost is typically high relative to the quantity and value of power produced from the low quality (i.e. low temperature) heat.